Method for introducing a cable into a cable guide tube

ABSTRACT

A method for installing a cable, more in particular a glass fibre cable, into a duct of a channelization system, the consideration which has led to this being based on the fact that the dragging working, considered from the cable inlet end, as caused by a flow of compressed gas effected in a relevant duct portion will increase according to a non-constant function and, dependent on the geometry and the length of the duct portion, can be insufficient over the initial part of it to compensate the friction forces exerted on the cable locally. According to the invention it has now been suggested to fulfill such a compensation function by exerting on the cable a pushing force working extending to the point where the dragging working will be sufficient to compensate the friction forces.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention generally relates to a method and device for introducing(installing) cables, such as e.g. optical glass fibre cables, into achannelization system of guide tubes, in general designated as "ducts",which are disposed, as a rule in the ground, according to apredetermined arrangement.

2. State of the art

According to a usual technique cables are installed by utilizing tensileforce. The course of the required tensile force, which has to remainsmaller than a permissible value in connection with the mechanicalproperties of the cable, depends on the geometry of the channelizationsystem (in which as a rule curves and/or windings occur), on thefriction forces between the cable and the duct, and on the properties ofthe cable. Without taking into account the stiffness of the cable thefollowing items are of importance for the required tensile force: thefriction forces consequent on the mass of the cable, and the frictionforces caused by the tensile stress occurring in the cable (inconnection with the curves and/or windings of a relevant duct portion).The former friction forces give rise to a tensile force which increaseslinearly with the length of cable installed. The latter friction forcesgive rise to a tensile force which increases exponentially with thenumber of curves or windings. This imposes an important restriction onthe maximum cable length which can be installed in one go. Because ofthe exponential increase of the required tensile force it can be triedto see to it that the cable tension will be as low as possible. Thisinvolves that the friction forces caused by the mass of the cable haveto be compensated locally. This means that the installation force has tobe exerted divided over the whole length of a cable section to beinstalled. In a method serving for that purpose a flow of compressed gas(compressed air) is effected from the inlet end of a relevant duct anddirected to the outlet end of the same, a relevant cable beingintroduced into the inlet end. A similar method is known from theEuropean patent No. 0108590. This patent also discloses a device foremploying such a method, which device is provided with a cable injectionunit with in it a hollow, substantially rectilinear cable lead-throughchannel with an entrance end and an exit end for leading in and leadingout a cable which has to be introduced into the relevant duct, whichcable injection unit is further provided with a gas channel, whichdebouches into the cable lead-through channel and via which compressedgas can be supplied to the cable lead-through channel, as well as with aset of wheels mounted opposite to each other and partly reaching intothe lead-through channel, and serving to move on a cable, disposedbetween these wheels and touching them, in the direction of the exitend. This known technique is meant for installing lightweight, flexibleoptical fibre cables. The flow of compressed air led through a ductexerts a drag force working on a cable introduced into the duct, due towhich such a cable will be dragged through the duct to its outlet end.As appears from this known technique the velocity of the flow ofcompressed air is practically linearly dependent (increasing) on thedifference in pressure there is between the inlet end and the outlet endof a relevant duct portion. The length of the duct portion over which acable section can be installed at one processing stroke is in this caselimited to abt. 200 m, even though it is expected in the above-mentionedpatent specification that said length can be extended to abt. 300 m fora cable with a weight of 3 gms/m and when the difference in pressure isabt. 55 psi. The two wheels forming part of a cable injection unitaccording to the aforesaid patent specification exclusively serve tocompensate the opposing forces exerted on the cable and caused by thedifference in pressure there is between the interior and the outerenvironments of the injection unit.

B. SUMMARY OF THE INVENTION

According to what is taught by the invention described in theabove-mentioned patent specification it is a requirement that the dragforces caused by the flow of compressed air are active from the inletend to the outlet end of a relevant duct portion. The object of thepresent invention is to improve the efficiency of the above-mentionedknown technique and to enlarge the possibility of using the same, withdue observance of the requirement that it must be possible to installthe cable into a channelization system in which curves and/or windingsoccur.

The consideration which has led to the present invention is based on thefact that the pressure gradient caused in a duct, and consequently thedragging working, is not constant, and, dependent on the length of arelevant duct portion, may be too small to compensate the frictionforces exerted on the cable at the beginning of the duct portion.Starting from this consideration a method according to the invention ischaracterized in that at said inlet end active pushing forces will beexerted on the cable over a certain length of the relevant duct.

A method according to the invention makes it possible to install, at oneprocessing stroke, a cable into a duct portion (with curves andwindings) over a length of abt. 700 m by utilizing the cable stiffnessof cables, more in particular optical fibre cables, used in practice.

A device for employing the method described above is according to theinvention characterized in that one of the wheels is coupled to apiston, which is movably mounted in a pneumatic cylinder, in such a waythat when compressed gas is supplied to the pneumatic cylinder,transverse forces will be exerted on the cable disposed between thewheels; and in that one of the wheels is coupled to a pneumatic motor,which is capable of providing a driving couple on the spot where arelevant cable is pressed between the wheels, which driving couple islarger than the driving couple which has to act on the cable tocompensate the difference between the pressure inside and the pressureoutside the cable lead-through channel.

It will offer an advantage when two or more cable injection units aremade to work in tandem connection. For that purpose a device accordingto the invention is characterized in that the cable injection unit iscomposed of two parts which are detachably fixed to each other, in sucha way that such a cable injection unit can be removed from a cableextending through the same.

For a further improvement of the efficiency a device according to theinvention is characterized in that the wheels have a hollow tread, sucha tread being provided with knurls substantially running parallel to theaxis of rotation of the relevant wheel, due to which the engaging effecton the cable will be considerably improved, also when such a cable isprovided with a lubricant and this is a special advantage when employingthe tandem method. For in this case a cable covered with a lubricantwill arrive at the inlet end of the cable injection unit disposed at theoutlet end of a relevant duct portion. In other words, a cable coveredwith a lubricant will then be led between the wheels.

When utilizing in practice a difference in pressure between the inletend and the outlet end of a duct of abt. 7.5 bar, the velocity of theflow of compressed air can have reached a very high value at the outletend (illustrative is a value of abt. 150 m/sec). For reasons of securityit has been suggested according to the invention to make use of acoupling unit disposed at the outlet end of a duct portion and (in thecase of tandem working) at the inlet end of the next injection unit.Such a coupling unit is characterized by a duct inlet for receiving aduct; by a compressed gas outlet pipe adapted either to lead away a flowof compressed gas coming in via the duct inlet at a high velocity to aplace which is safe for operating personnel or to slow down the velocityof such a flow to a safe velocity; and by a cable outlet opening with alining capable of slowing down particles which may come along with thecable and/or the flow of compressed gas, which outlet opening is coveredby a cover plate, which is pivotally mounted on the coupling unit andsucks itself, which cover plate can also be pushed aside by the cable ina simple way.

Summarizing the invention results in the following advantages:

quick and simple installation without the need of a pulling rope;

small cable strain during the installation process;

relatively long length of installation per injection unit; and efficientand simple tandem working, the relevant injection units being capable ofworking in series with intermediate distances which are practicallyindependent of the route of the channelization system.

C. BRIEF DESCRIPTION OF THE DRAWING

The invention will be further explained hereinafter with reference tothe drawing in which

FIG. 1 shows a set of diagrammatic representations with the aid of whichthe essences of the method according to the invention will be explained;

FIG. 2 shows a diagrammatic representation of a duct portion withcontained in it a cable section on which a dragging working caused by aflow of compressed gas is exerted;

FIG. 3 shows a diagrammatic representation to illustrate the pushingforce working which, according to the invention, is exerted on thecable;

FIG. 4 shows a diagrammatic cross-sectional view of an embodiment whichis illustrative of a device for employing the method according to theinvention; and

FIG. 5 shows a perspective view of a possible embodiment, taken apart,of a cable injection unit according to the invention.

D. REFERENCES

[1] European patent 0 108 590.

[2] "A radically new approach to the installation of optical fibre usingthe viscous flow of air" by S. A. Cassidy et al. in Proc. IWCS (1983)250.

E. DETAILED DESCRIPTION OF THE FIGURES

The essence of the method according to the invention will be furtherexplained in the first instance with reference to fIG. 1.

An important consideration underlying the present invention implies thatthe course of the pressure in a duct portion having a length 1 and apressure at the beginning respectively the end of this portion of p(o)respective p(l), is a non-linear function of the place (x) and can berepresented for an isothermal flow by ##EQU1## so that the pressuregradient which is a measure for the dragging working exerted on thecable by the flow of compressed gas can be written as ##EQU2##

It is noted that what is expressed by this formula (2) in fact appliesto an "empty" duct portion. When such a duct portion contains a cable,the whole will become considerably more complicated. However, inpractice it has appeared that what is expressed by the formula (2) is auseful valuation.

With the aid of the technique described in reference [1] it will bepossible to compensate the friction forces, caused by the mass of thecable, locally. According to this known technique it is assumed, andthis also appears from reference [2], that the course of the pressurebetween the beginning and the end of a duct portion is linear. In otherwords, according to these known techniques the pressure gradient##EQU3## considered over the whole length 1 of the duct portion will beconstant. It is also derived that the hydrostatic force per unit oflength F/1 (dragging working caused by the compressed air blown into theduct portion) can be represented by ##EQU4## See in this connection FIG.2, in which a duct portion with a length 1 and a cable contained in itare diagrammatically shown. The arrow represents the flow of compressedair blown along the cable.

The friction force ##EQU5## to be compensated per unit of length can berepresented by ##EQU6## in which f stands for the coefficient offriction between the cable and the duct, and W for the cable weight perunit of length.

The diagrammatic representations of FIG. 1 show the course of thepressure gradient ##EQU7## as a function of the place x along a ductportion, it being assumed that p(o)=8.5 bar and p(1)=1 bar (absolute).The diagrams in this figure are drawn for illustrative duct portions ofa length of 437 m respectively 782 m and for a1 and a2 it is assumedthat the pressure gradient will be a constant ##EQU8## whereas for b1and b2 it is thought that the pressure gradient will not be constantaccording to the formula (2). Moreover, on the basis of the formulae (3)and (4) the arrow on the right indicates the pressure gradient ##EQU9##which is indispensable to compensate the friction force exerted on thecable (f=0.25; W=0.76 N/m; r_(d) =13 mm; r_(k) =4.85 mm). In the casesa1 and a2, where the pressure gradient is assumed to be a constant, ithappens that when the duct length l=782 m, the pressure gradient willjust be sufficient to compensate the friction forces, whereas thispressure gradient will be larger than necessary for such a compensation,when the duct length l=437 m.

From the course of the curves b1 and b2, which are illustrative of theformula (2), the following appears:

for the duct length l=437 m the pressure gradient considered over thewhole duct portion will always be sufficient and near the inlet unitjust sufficient to compensate the friction forces exerted on the cable;

for the duct length l=782 m the pressure gradient considered over arelatively large part of the duct portion will not be sufficient tocompensate the friction force exerted on the cable locally.

From the above it can be concluded that the above-mentioned techniquewill be insufficient to install per injection unit a cable length longerthan a certain limiting value, in the aforesaid case 437 m, into a duct.In other words, on the basis of the assumption made in connection withthe case a2 and the possible installation length calculated from it, itwill not be possible to install a cable into a duct portion with such alength by making use of only a flow of compressed gas. According to thepresent invention it has now been suggested to exert, from the inlet endof a relevant duct portion and by making use of the stiffness of a cableto be installed, a pushing force working on such a cable. If somewherein the duct the dragging working of the flow of compressed gas blowninto the same is still insufficient to compensate the friction forces,this function will be fulfilled by such a pushing force working. In thisway the length of the duct portion over which a cable can be installedby means of only one injection unit can be considerably extended, whichis also established by experiment. A factor of two with regard to aknown blowing-in technique has proved to be possible. Illustrative of acable used in practice is a cable with a stiffness of abt. 0.9 Nm². Sucha stiffness will be sufficient to hinder "buckling" of the cable whenpushing it into a duct in such a way that the cable will not pressitself fixed against the wall to any extent, and on the other hand thefriction forces particularly arising consequent on the stiffness of thecable in curves and/or windings in the relevant duct portion still proveto be compensable. By employing the method according to the invention ithas proved to be posssible to install a cable into a duct portion with alength of more than 700 m. Briefly summarized the method according tothe invention results in the following advantages:

quick and simple installation,

small cable strain during the installation process;

considerable extension of the length of installation, which can beeffected by means of only one injection unit; and

several injection units can be used in series (tandem connection) in anefficient and simple way.

Illustrative of the invention is the following example:

The duct portion in this example has a length of 667 m and an insidediameter of 26 mm, and right-angled curves at distances of 150, 250, 400and 600 m from the cable inlet end, which curves have a radius ofcurvature of 1 m; moreover, the duct portion has a winding course with aperiod of 4 m and an amplitude of 5 cm. The cable to be installed has adiameter of 9.7 mm, a weight of 0.65 N/m and a stiffness of 0.9 Nm². Thecoefficient of friction between the cable and the inner wall of the ductis 0.25. Compressed air supplied by a compressor with a capacity of 75l/sec (atmospheric) and a maximum working pressure of 7.5 bar(overpressure) is used for installing the cable. By means of the formula(2) a value of 5.34* 10⁸ /p p_(d) /m can be calculated as an estimationfor the pressure gradient. Only after 386 m this pressure gradient willbe sufficiently large to stop the effective friction force fW of 0.19 Nm(consequent on the existing curves and windings the effective cableweight per unit of length has become larger due to the stiffness of thecable) exerted on the cable locally. The course of the necessary pushingforce F has been numerically calculated as a function of the distance x(from the inlet end of the duct portion) as illustrated in FIG. 3. Ithas appeared that the cable can be installed into the portion inquestion by means of the combination of such a pushing force working anda flow of compressed air.

For the sake of completeness it is noted that the various negativeeffects (curves hinder the development of pushing force) and positiveeffects (moving the position where the pushing force is zero because ofthe fact that in the part after it the compressed air exerts aneffective dragging working) can play a part. Moreover, a change of thegeometry can have considerable consequences. For example, if the numberof windings and/or curves is decreased, the length of the duct portioninto which the cable can still be installed by means of the combinationof a flow of compressed air and a pushing force working, will be longer.

FIG. 4 is illustrative of an embodiment of a device for employing themethod according to the invention. More in particular FIG. 4 shows adiagrammatic cross-sectional view of such a device. A device of thattype or a cable injection unit is in its generality designated by 1.This unit comprises a housing 2, in which a substantially rectilinearcable lead-through channel is formed, which channel has an inlet end 4through which a cable can be led into the injection unit, and an outletend 5, which has been adapted to be coupled to a relevant duct 6, intowhich a cable has to be installed, while at the same time forming agastight closing. An inlet tube debouching into the lead-through channeland meant to be connected to a source of compressed gas (a compressorwhich is not shown in the figure) is designated by 7. In the case of ausual flow resistance of the relevant duct portion and a conventionalcompressor with a capacity of 75 ltr/sec and a maximum working pressureof 7.5 bar (overpressure), a flow of compressed gas (flow of air) in theorder of magnitude of 75 ltr/sec will stream via said inlet tube. A setof wheels 8, 9, 10 and 11 is pivotally mounted in a housing and partlyreaches into the lead-through channel. The set of wheels 8 and 9 issupported by a frame 12, which is pivotally coupled to a piston rod of apiston 13, which is movably mounted in a pneumatic cylinder 14. Asdiagrammatically shown in FIG. 4, the two wheels 8 and 9 can be setturning via a transmission mechanism by means of a pneumatic motor 15,which is mounted on the housing. Opposite to the embouchure of the gasinlet tube the cable lead-through channel is bounded by a streamlinedsmall pipe 16, which ensures that the course of a cable disposed in thelead-through channel will remain substantially rectilinear, in spite ofthe strong flow of compressed gas. In other words, the cable will beprevented from being blown to a "buckling" in that portion, which wouldseriously hinder the introduction of the cable into the duct. A cableled into the lead-through channel via the inlet opening 4 forms via adiagrammatically drawn washer 17 a gastight closing. In consequence ofthe compressed air supplied there will be a difference in pressurebetween the interior and the exterior of the housing. Owing to this aforce working will be exerted on the cable disposed in the lead-throughchannel, which takes the opposite direction with regard to the desireddirection of movement of this cable. When compressed air is supplied tothe cylinder 14 and to the motor 15, this "oppositely directed" forceworking will be compensated. A pneumatic motor has the advantage thatthe driving couple provided by it is proportional to the pressure causedin the housing; moreover, when compressed air is supplied, a pneumaticmotor can be slowed down to a standstill without harmful consequences,and also be kept in such a state (if desired for a long time). Thelatter is a particular advantage when several cable injection units areused in series (or in tandem connection).

Within the scope of the present invention, however, the pneumatic motoris considerably more powerful than necessary for compensating theabove-mentioned "oppositely directed" force working. Illustrative ise.g. a motor capable of providing a force working which is thrice aslarge as the force working necessary for the above-meant compensation.By means of a motor thus dimensioned it will be achieved that over alength area extending over a certain distance from the beginning of therelevant duct portion, a pushing force working is exerted on the cabledisposed in the duct. As explained with reference to FIG. 1, such apushing force working neutralizes the local friction force working,caused by the friction between the cable and the inner wall of the duct,and by the weight of the cable, at those places where the pressuregradient respectively the dragging working caused by the flow ofcompressed air is still too small to compensate the friction forceworking referred to. In order to ensure that the pushing force workingwill be effective, it is essential that the relevant cable has a certainstiffness. The cables used in practice meet this requirement. Inpractical situations it has appeared that the length of the duct portionover which a cable can be installed by means of only one injection unit,can be extended by a factor of abt. two, when use is made of such apushing force working. The speed at which the cable is introduced into aduct can be regulated by means of a pressure regulator (not shown inFIG. 4). To promote the engagement between the wheels and the cable tobe moved on by them, each of these wheels has a hollow tread, which isprovided with transversal knurls. The advantage of such a constructionis that said wheels cannot get filled up with material from the cablesheath and pollutants, if any, taken along by the cable, and thatslipping, even when the cable sheath is covered with a lubricant, willbe effectively hindered.

In the embodiment represented in FIG. 4 two driven wheels and two"counter pressure" wheels are used. However, if need be, also anembodiment with one respectively more than two driven wheels and onerespectively more than two counter pressure wheels can be made use of.In the latter case it is advisable to use at least two pneumaticcylinders, each of the pneumatic cylinders operating two wheels at most,which are in this case mounted in a frame, which is pivotally coupled tothe relevant piston rod. The pneumatic cylinder/piston combinationserving to cause the pressing working has the advantage that in the caseof a certain supply of compressed air the pressure force working exertedon the cable will be substantially constant. That means thatnotwithstanding variations in the thickness of the cable, this pressureforce will remain constant. Moreover, an upper limit corresponding tothe maximum working pressure of the supply compressor of 7.5 bar, hasbeen set to the pressure force working exerted. This means that thepermissible squeezing pressure of a cable used in practice will not beexceeded when using the pneumatic cylinder/piston combination.

By means of the above-described construction the motor can be brought toa standstill by slowing down and bringing to a standstill the cablemoved on by that motor.

FIG. 4 also shows a coupling or closing unit 18. Such a unit comprises ahousing 19, which leaves space to a room 20, which is connected to theouter environments via an inlet opening 21, an outlet opening 22 and acompressed gas outlet pipe 23. The inlet end has been adapted to form agastight coupling with the final part of a duct portion 24, at thebeginning of which use is made of a cable injection unit similar to theone shown in FIG. 4. The velocity at which the compressed air flows intothe room 20 via the inlet end 21 is substantially determined by themagnitude of the flow of compressed air. In the case of values of abt.7.5 bar and 75 ltr/sec this velocity will be in the order of magnitudeof 150 m/sec. For reasons of security and for the protection of aninjection unit located near the outlet opening 22 the compressed airoutlet pipe has been designed in such a way that the compressed airsupplied will be slowed down, via a widening initial part 25, withoutcausing whirls, and led away, via this outlet pipe 23, to a place whichis safe for the operating staff. As an alternative said outlet pipe canhave been dimensioned in such a way that the velocity at which thecompressed air streams out has been reduced to a safe value. The cable26 installed in the duct portion 24 can be led to the next cableinjection unit, in this case the injection unit 1, via the outletopening 22, which is provided with a lining 27' capable of slowing downquick particles which may come along with the cable and/or the flow ofcompressed gas. As a rule the opening 22 is closed by means of a valve27 pivotally mounted on the housing 19, which valve will attach itselffirmly to the outside of the housing due to the acceleration of the flowof air there, when there is no cable at that place, in such a way thatthe valve will offer protection against the particles moving on at ahigh speed. By means of the cable supplied this valve can be opened, inwhich case the lining 27' will then fulfil the function of the valve 27.After that this cable can be installed via the injection unit 1 into theduct portion 6, which follows after the duct portion 24. For thatpurpose the two injection units have to work in series or in tandem. Inconnection with such a tandem working each cable injection unit andcoupling or closing unit which has to be used in that process, has beendesigned as two parts which are detachably fixed to each other. This isillustrated in FIG. 5, in which these parts 38 and 39 are detached fromeach other. In this figure the relevant parts are indicated as follows:

2; 2¹ component parts of the housing;

3 cable lead-through channel;

4; 4¹ component parts of the inlet end;

5 outlet end (one half of it);

6 duct;

7 compressed air inlet tube;

8 wheel with hollow tread provided with transversal knurls;

13 cable pressure mechanism;

15 pneumatic motor;

16 streamlined pipe (one half of it);

17 recess for one half of a washer;

26 cable;

28 on/off switch compressed air;

29 reducing valve for controlling the speed of the pneumatic motor;

30 pressure gauge for measuring the pressure supplied to the pneumaticmotor (indicative of the motor torque);

31 reversing switch for reversing the direction in which the motor runs;

32 pressure gauge for measuring the pressure at the entrance of the ductportion 6;

33, 33¹ fixing clamps for detachably fixing the parts 38 and 39 to eachother;

34 centring bosses.

For the sake of completeness it is noted that in the embodimentaccording to FIG. 5 the motor 15 drives only one wheel 8, which wheelcan co-operate with a wheel (not shown in FIG. 5) mounted in the part 39opposite to the former wheel. Moreover, the drive mechanism 13 in thisembodiment is provided with a pressure spring (not shown in the figure),due to which the latter wheel is pressed against the cable. It isobvious that this pressure mechanism can be replaced by the constructioncomprising a pneumatic cylinder/piston combination and described inconnection with FIG. 4.

The special construction of the wheels described in what precedes makesit possible that a cable disposed between them will be moved oneffectively without causing damage to the cable sheath, and no matterwhether such a cable sheath is covered with a lubricant or not. Thelatter is an advantage when the tandem method is employed. For in thatcase a cable provided with a lubricant is supplied to an injection unit.For a further improvement of the working of the wheels it may beadvisable to provide the outer sheath of a cable to be installed withcorresponding transversal knurls. These knurls may also result in anenhancement of the dragging working caused by the flow of compressedgas.

We claim:
 1. A method of advancing a longitudinal element along apreviously installed tubular pathway in situ, said tubular pathwayextending between two points, said method comprising(a) inserting andadvancing the free end of the element into the pathway by applying tosaid element a force in the direction of insertion; (b) inserting agaseous medium into the pathway with a flow velocity much higher thanthe desired rate of advance of said element through said pathway; and(c) propelling the free end and following portions of the element alongthe pathway by continuance of said insertion force in cooperation with afluid drag force of said gaseous medium passed throughout the length ofthe pathway in the desired direction of advance such that said dragforce on the element is distributed throughout the length of saidelement then present in the pathway,characterized in that thelongitudinal element is a cable with an intrinsic stiffness and that theinsertion force is exerted dependent on the intrinsic stiffness of saidcable, as an at one point concentrated pushing force which is effectiveover a main part of the pathway to complete a deficiency in frictionforces compensating the effect of said drag force in said main part,said drag force and said pushing force enabling in their cooperation thecable and its advancing free end to be advanced past bends, which may bepresent in the pathway,without unacceptable pulling or pushing forcesbeing concentrated on any portion of the cable, said gaseous mediumhaving substantially higher velocity than said cable throughout thelength of the pathway.
 2. A method in accordance with claim 1characterized in that said pushing force is a few times larger than theforce required for overcoming the pressure difference existing at theinsertion point of the pathway between the inside and outside of thepathway as a consequence of said insertion of the gaseous medium.
 3. Amethod in accordance with claim 2 wherein forces transversely exerted onthe cable are made use of to cause said pushing force, characterized inthat for these transverse forces a value has been chosen which is of thesame order of magnitude as, but smaller than a transverse force valuecorresponding with the maximum squeezing pressure permissible for therelevant cable.